The present invention relates to a PSK (Phase Shift Keying) demodulator, and more particularly to a PSK demodulator having an automatic frequency control (AFC) circuit for stabilizing the recovered carrier.
In a conventional PSK demodulator of this type, the carrier wave is recovered from an input modulated wave consisting of, for example, an N (N=2, 4, . . . ) phase PSK wave, by a carrier recovery circuit. The recovered carrier wave is stabilized by an AFC circuit, and is applied to a phase detector (or coherent detector) which also receives the input modulated wave form and which processes the two signals to derive as an output the demodulated signal. There are two versions of the carrier recovery circuit: one of frequency multiplier type usually comprising an N frequency multiplier, a band-pass filter (BPF) and a 1/N frequency divider, and the other of inverse modulator type usually comprising a delay circuit and an N phase modulator.
The AFC circuit in such a demodulator, though working on the recovered carrier wave, does not correct phase errors that may result from frequency fluctuations in the circuits outside the AFC circuit loop, such as errors which appear in the BPF or delay circuit of the carrier recovery circuit described above. Therefore, in a transmission system where the carrier frequency of the input modulated wave fluctuates greatly, there can arise significant performance deterioration of the demodulator owing to the phase errors occurring outside the AFC circuit loop.